Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the subject of the invention is a method of producing the medium manganese ferritic-austenitic steel with a lath structure, especially for forgings.
• High-strength medium manganese steel is intended especially for forgings with increased ductility and crack resistance.
• the method of production and chemical composition of medium manganese steels intended for forgings is presented in the publication [ A. Gramlich, R. Emmrich, W. Bleck, Austenite reversion tempering-annealing of 4 wt.% manganese steels for automotive forging applications. Metals 9, 2019, 575 ].
• the subject of the research were steels containing 0.15-0.19% C, 4% Mn, 0.5% Si, 0.02-0.2% Mo, 0.02-0.03Ti, 0.035% Nb and 0.0005 -0.005% B. Before the forging process, the steel was homogenized at a temperature 1200°C for 5 hours. Then, the forging process was carried out to produce rods, which was finished with air cooling.
• the steel with a martensitic structure was obtained.
• the rods were heated to a temperature of 600-675°C and held at this temperature for 1 hour to ensure the diffusion of carbon and manganese from ferrite to austenite.
• the steel with a ferritic structure containing retained austenite and some martensite fraction was obtained.
• a method of obtaining thin sheets with a structure composed of ferrite and retained austenite was shown in the Dutch patent application US2023010877 (A1 ).
• the steel ingots are heated to a temperature of 1150°C for at least 1 hour, after which hot rolling is carried out.
• the sheets are heated to an intercritical annealing temperature (between A c1 and A c3 ) below 700°C and they are held at this temperature for at least 5 hours, however heating times above 10 hours are preferred, in order to effectively enrich the austenite with manganese; then the steel is cooled in air or water.
• the intercritical annealing process of medium manganese steels is mainly used for semi-finished products in the form of sheets. So far, the existing material solutions concerning steels with a structure composed of ferrite and retained austenite intended for forgings do not contain aluminum addition, which together with silicon prevents the formation of cementite and additionally increases the intercritical range (the difference between A c1 and A c3 temperatures). Increasing the Mn content above 4 wt. % causes a reduction in the A c1 temperature, which allows for performing intercritical annealing at lower temperatures, which saves energy used in the heat treatment process. Moreover, the high hardenability of steel allowing to obtain the martensitic structure during air cooling was achieved in existing solutions by addition of Mo and/or B to the steel, which increases production costs.
• the aim of the invention was to design the chemical composition of medium manganese steel and the time-temperature parameters of heat treatment applied after forging, which will allow to obtain the structure composed of ferrite and retained austenite, uniform in the cross-section of the forging showing high strength, ductility and crack resistance.
• the aim of the invention is a method of producing medium-manganese ferritic-austenitic steel with a lath structure carried out by austenitization, hot forging, cooling and heating, characterized by the fact that the initial material with a composition of 0.15-0.2% mass. C, 4.5-5.5% mass. Mn, Al and Si, with the total content of Al and Si not exceeding 2% mass, and the rest is Fe, is austenitized at a temperature of 1100-1150°C, then hot forged at a temperature of 950-980°C during the last deformation step during forging, then cooled in air to the room temperature, then heated to a temperature of 680-700°C and held at this temperature for 30-60 min, and then cooled in air to room temperature.
• Mn addition at a concentration of 4.5-5.5% mass increases the hardenability of steel, which allows to use air cooling after hot forging without the need to add other chemical elements increasing hardenability, such as molybdenum or boron.
• Manganese is a much cheaper chemical element than Mo or B, which reduces the production cost of forgings. Due to the Mn addition, it is possible to obtain a homogeneous structure on the cross-section of forgings with different thickness.
• Mn is an austenite-stabilizing element, which allows to obtain more than 20% of austenite in the structure.
• a low carbon content in steel: 0.15-0.2% weight has a positive effect on the impact strength and machinability of forgings.
• the limited C content has also a positive effect on durability of forging tools.
• Al and Si alloying additions whose total content does not exceed 2 wt.% prevent the formation of cementite, which reduces the fraction of retained austenite in the microstructure of steel and has an unfavorable effect on the impact strength.
• Aluminum additionally influences the expansion of the intercritical range to min. 200°C.
• the A c1 temperature of the medium manganese steel according to the invention is not higher than 670°C and the temperature range between A c1 and A c3 is min. 200°C.
• the medium manganese steel obtained according to the invention shows the following structural composition: less than 2% of fresh blocky martensite, more than 20% of retained austenite in the form of films with an average thickness not exciding 300 nm and a mass content of C min. 0.4 wt.% and Mn min. 7.0 wt.%.
• Retained austenite in the form of thin films prevents the initiation of microcracks, while martensite formed as a result of plastic deformation contributes to blocking the propagation of possible microcracks.
• the content of alloy additions in the steel according to the invention is set to achieve a specific hardenability and an A c1 temperature not higher than 670°C and the temperature range between A c1 and A c3 should be at least 200°C.
• the chemical composition of the steel according to the invention is additionally subjected to the condition that the hardenability allows to obtain the martensitic structure during air cooling to the room temperature applied after hot forging.
• the chemical composition of steel should be designed to avoid the occurrence of ferritic, pearlitic and bainitic transformations during air cooling applied after forging.
• the key structural constituent of steel is ductile retained austenite showing high stability determined by a carbon content min. 0.4% mas. and Mn min. 7% mas. in the form of thin films with a width less than 300 nm, which relaxes the stress or gradually transforms into martensite under operating conditions of the forging, preventing the initiation of cracks and blocking their possible propagation.
• the key element in designing the time-temperature parameters of the intercritical annealing process is to minimize the amount of blocky retained austenite, which initiates cracking during the operating conditions of forgings.
• the structure of medium manganese steel according to the invention ensures high mechanical properties compared to existing solutions.
• the chemical composition of the steel according to the invention allows obtaining retained austenite in the structure with the following parameters:
• An ingot with a chemical composition of 0.19C-5.4Mn-0.87Si-1.0AI with a cross-section of 100x100mm and a weight of 100 kg was produced using a vacuum furnace in an argon atmosphere. Then, the ingot was initially forged into a rod with a diameter of 80 mm. The forging process was preceded by austenitizing the ingot in a furnace at a temperature 1150°C for 60 minutes. The same austenitizing parameters were used during the next hot forging cycle, which was carried out in a press in two deformation steps at the following temperatures: 1100°C (I) and 980°C (II). Then, the forging was cooled in air to the room temperature.
• Fig. 1 shows the CCT diagram of the steel according to the invention from the melt described as IA.
• Fig. 1 shows that for a wide range of cooling rates 0.05-60°C/s, martensite was observed in the structure of steel.
• the distribution of HV hardness in Fig. 1 shows that a high hardness from 438 to 496 HV was obtained in the range of tested cooling rates, which proves the high hardenability of the steel.
• the temperature difference between A c1 and A c3 is 227°C.
• Fig. 2 shows the morphology of retained austenite in the form of thin layers with a thickness not exceeding 300 nm.
• the microstructure contains ferrite and retained austenite in the amount of 20%, estimated using the X-ray diffraction method.
• Fig. 3 is a TEM-EDS image showing differences in a Mn content (wt.%) in ferrite and films of retained austenite after the intercritical annealing process.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Crystallography & Structural Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Heat Treatment Of Steel (AREA)

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• 2023
  • 2023-12-19 EP EP23020561.9A patent/EP4575007A1/de active Pending

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